Heretofore, natural diamond, synthetic diamond grains produced from carbonaceous substances as raw material by the ultra-high pressure method and having their auto-morphic shape, grains obtained by grinding such diamond have been generally used as diamond grains for grinding and abrasion.
Recently, the vapor-phase method for synthesis of diamond has been developed. Various procedures for carrying out this method have been disclosed.
The inventors formerly filed PCT/JP 88/00299 relating to a composite diamond grain composed of a ceramic or metal grain having a diameter equal to or less than 30 .mu.m as a nucleus and a vapor-phase method crystal diamond enclosing the grain therewith. The nucleus of the composite diamond grain involved in said application has a diameter of 30 .mu.m at most and has conical projections and rectangular faces and square faces on the grain surface and, therefore, has restrictions on use. To expand the uses of the composite diamond grain, the inventors have continued a study in search for a method capable of producing a composite diamond possessing a nucleus no less than 30 .mu.m in diameter and have accomplished the present invention.